Typically, in the rotary drilling of a well, one fluid is used to drill the well, a drilling mud, and another fluid, a cement slurry, is used to cement the casing or liner into the well. For simplicity, the term "casing" will be used to refer to both the well casing and the well liner.
The drilling mud, consisting of liquids and solids, is circulated down the drill pipe, past the bit, and back to the surface through the annulus between the drill pipe and the borehole. The drilling mud functions primarily to clean the borehole by suspending and lifting formation cuttings to the surface, to cool the drill bit, to lubricate the drill string, and to seal the formations penetrated by the drill bit to prevent the escape of fluids therefrom. Throughout the drilling operation, the proper functioning of the mud is constantly monitored by controlling the mud weight, viscosity and gel strength, and controlling filtrate loss.
The weight of the drilling mud is important in preventing a blowout. The mud density must be high enough to counterbalance the pressure of the subterranean fluids with the hydrostatic head of the drilling mud in the well. Mud density is controlled primarily by regulating the solids content of the mud. In weighted muds, a material, such as barite, is added to increase the mud weight and keep it within tolerable limits.
The drilling mud must have a viscosity which permits circulation at high rates without excessive pump pressures. On the other hand, the drilling mud must be viscous enough to carry the drill cuttings to the surface. Closely related to the viscosity is the mud gel strength. The gel strength is indicative of the mud's tendency to thicken during periods of interrupted circulation. It must be high enough to suspend the barite and drilling chips and prevent them from settling during periods in which circulation of the mud is interrupted. If the gel strength is too high, it may be difficult to reestablish circulation. Moreover, when pulling drill pipe from the well, a high viscosity or gel strength results in a swabbing action which reduces the pressure in the well and may possibly result in a blowout. The viscosity and gel strength may be increased by adding bentonite, flocculating the clay solids or adding specially formulated additives. The viscosity and gel strength may be reduced by diluting with water or adding chemical thinners. Generally, most commonly used viscosifiers tend to increase the attraction between suspended particles while thinners tend to reduce the attraction.
Controlling filtrate loss is another important function of the drilling mud. When the borehole enters a permeable formation, the filtrate flows into the formation. To control the filtrate loss, the drilling mud should contain particles of varying size which will form a filter cake on the wall of the borehole. The filter cake should have a low permeability so that it will be relatively thin and will not impede the raising, lowering or rotating of the drill pipe.
Typically, after the well has been drilled to a desired depth, a permanent casing is strung in the hole and cemented in place. Cementing is accomplished by pumping a cement slurry down the casing and up the annular space behind the casing where it is allowed to set, thus bonding the casing to the formation. The primary functions of cementing are to seal the formation and the wellbore to prevent escape and communication of subterranean fluids and to protect the casing. As with the drilling mud, the cement slurry must have the proper density, viscosity and filtrate loss properties.
An additional consideration is the cement setting time. If the setting time is too short, the cement may set before it is properly in place. On the other hand, the setting time should not be any longer than necessary because of the delay in drilling activity.
Still another consideration in cementing operations is to obtain a complete displacement of the drilling mud and filter cake in place in the well annulus by the cement slurry. A primary problem in the cementing operation is the effective removal of drilling mud and filter cake during cementing operations. Inadequate displacement of the drilling mud and filter cake will leave channels of gelled mud and filter cake remaining in the annulus after the cement is in place. The channels of mud and filter cake remaining in the annulus may greatly reduce the effectiveness of the cementing operation. Moreover, the mud and filter cake may contain components which contaminate the cement slurry as it displaces the mud and consequently delay or prevent the setting of the cement, or reduce the strength of the set cement. Further, a thick filter cake deposited by the mud on the borehole walls presents an inadequate surface for cement bonding and an inadequate barrier to prevent communication among permeable strata separated by impermeable strata.
Usually, different fluids are used as drilling muds and cement slurries. Several problems have been associated with this practice. Mixing between the cement and the mud as the cement is put in place may have deleterious effects on the cement. Inadequate displacement of the mud or filter cake by the cement may result in an ineffective or shortlived seal between formations. Delay in changing from drilling mud to cement may result in the mud gelling, exacerbating the problems of in-place cement-mud mixing and cement channeling. Moreover, using separate fluids for the drilling mud and the cement slurry may require the additional use of scarce resources, such as water, and pose the additional problem of disposing of the drilling mud.
One attempt at eliminating the problems of using two separate fluids for drilling and cementing is disclosed in U.S. Pat. No. 3,168,139 issued to Kennedy in which a drilling mud is converted to a cement slurry by mixing the cement and water with the aqueous drilling mud at the surface prior to its recirculating down the borehole. While the drilling mud is used as a component of the cement slurry, the mud containing cement slurry cannot be further used as a drilling mud once the cement has been added since the mud containing cement slurry cannot be prevented from setting. Furthermore, the filter cake does not contain the newly added cement and cannot be set. Therefore, while the problems of cement-mud mixing and mud channeling may be reduced by Kennedy's method, they are not completely eliminated.
Another attempt at using the drilling mud as a component in the cement slurry is disclosed in U.S. Pat. No. 3,499,491 issued to Wyant. This patent discloses the combination of drilling mud with hydraulic cement and sodium silicate glass to form a mud concrete. Like the procedure of Kennedy, the Wyant patent requires the addition of cement and additives to the drilling mud to make a cement slurry which thereafter is no longer useful as a drilling mud.
A chemically convertible drilling fluid is disclosed in U.S. Pat. No. 3,557,876 issued to Tragesser. This patent discloses a drilling fluid comprising water, clay and pozzolan which is convertible to cement at the conclusion of the drilling operation by admixing therewith an alkaline earth hydroxide. U.S. Pat. No. 3,605,898 issued to Harrison discloses an aqueous hydraulic cement slurry containing heptolactone which may be used as a well circulation fluid. This cement composition requires the addition and admixture therewith of a polyvalent metal salt, such as calcium chloride, to promote setting. As with the other attempts, both Tragesser's and Harrison's cement require the mixing of a chemical additive therewith and the problems of in-place cement-mud mixing and mud channeling are not totally eliminated.
In contrast, the present invention does not require the admixture of any additional components to the drilling fluid to effect conversion of the fluid to a settable cement slurry, and the problems of in-place cement-mud mixing and mud channeling are eliminated. Further, since the fluid of the invention is convertible to cement without the requirement for subsequently admixing a separate hardening component, the filter cake deposited on the formation face by the fluid during its use as a drilling mud will also be converted to cement, thus an effective seal of the formation will be accomplished and a surface provided to which the bulk body of the cement may be strongly adhered.